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A SOS story about Sandra Alvarenga, a working mother who wants enough money to pay for jaw surgery for her son William, 10. At home in San Rafael with his favorite snack, Cheetos. Photographer:� Eric Luse / The Chronicle MANDATORY CREDIT FOR PHOTOG AND SF CHRONICLE/ -MAGS OUT

The holiday season is in full swing and with it the inevitable requests for charity -- a warm coat, a dollar bill for the Salvation Army Santa, toys for underprivileged children.

But when was the last time you had a chance to buy someone a smile for life?

That's what Sandra Alvarenga, single mother of two, is wishing her 8-year-old son, William, could have. Last summer, he was diagnosed with a jaw disorder that crowds his front teeth together and eventually will push them on top of one another.

He needs $3,000 worth of orthodontia, including monthly visits to a doctor and a device he must wear in his mouth that will gradually expand his jaw to make enough room for his teeth.

"There's no way on earth I can come up with $3,000, and I don't even have the credit cards," his mother said this week.

William also will eventually need braces, too.

"And of course, my insurance won't pay for that," Alvarenga said.

One orthodontist offered to accept a down payment of $800 and then monthly installments, but that is more than Alvarenga, who works as a legal assistant at a nonprofit group, can afford:

"Not with my car payment, my other bills."

At her family's tiny apartment in San Rafael, a Christmas tree decorated by the kids and cheery family photos suggested a loving household. But the temperature in the apartment, somewhere in the 50s, indicated the Alvarenga family was just barely getting by.

Asked about the heat, Alvarenga said simply, "We try not to use it."

Her rent just went up $130 a month, meaning there will be more things she and her children will have to live without.

Alvarenga doesn't have other family to turn to. Her mother is busy caring for Alvarenga's disabled younger sister.

"My kids have been pretty much aware of what's going in my life," she added. "They understand when we can't have things. We try to hang in there. Sometimes we have to rob Peter to pay Paul."

On top of the family's meager finances, Alvarenga has been battling her children's father in court over their custody. She said her kids had needed therapy to deal with the trauma.

None of it seems to bother William, a friendly second-grader at Galinas Elementary School who enjoys playing as a linebacker for the Central Marin Wildcats and is hoping for an Oakland Raiders football helmet for Christmas.

During an impromptu photo shoot he smiled repeatedly, snacked on Cheetos and played with his 10-year-old sister's pet bunny, Eddie.

Alvarenga said she's hoping William will get his jaw fixed before his condition worsens and he requires expensive, painful surgeries.

"I would have to spend double the amount than what could be spent now," she said.

Just off the kitchen hangs a picture of Alvarenga, her daughter, Emily, and William. Everyone is all smiles. With help, Alvarenga hopes she can keep it that way for her son.

"I'm sure down the road he's going to appreciate all the sacrifices we've made," she said.

Donations to the Season of Sharing Fund help thousands of people in the Bay Area throughout the year. Assistance is in the form of grants paid directly to the supplier of services, such as a landlord. Individuals cannot receive direct grants.